Spark plugs have been used to ignite, for example, the fuel-air mixture in the combustion chamber of an internal combustion engine. A spark plug includes, for example, a central electrode and a ground electrode and ignites the fuel-air mixture by spark discharge caused in a gap between the central electrode and the ground electrode. A spark plug includes an insulator that insulates the central electrode and the ground electrode with each other. An example of such an insulator is made of a material containing alumina.